This invention relates in general to digital communications systems and more particularly to synchronization of digital information in a digital communications system for coherent demodulation of continuous phase modulation signals.
Frequency synchronization is essential for reliable digital communication between the transmitting (Tx) and receiving (Rx) radios. As is known in the art, both transmitter and receiver should have the same nominal frequencies when communicating together. In practice the reference oscillators in the two radios (Tx and Rx) have different errors from the nominal frequency. Therefore the receiver needs to xe2x80x9ctunexe2x80x9d within a certain tolerance of the actual transmitter frequency to receive information. This is commonly known as frequency synchronization. In particular, communication systems using coherent demodulation methods are highly sensitive to the frequency difference between the Tx and Rx radio frequencies. An automatic frequency control system (AFC) is required to bring and maintain the frequency error to within the tolerance allowed by the modulation scheme.
Most mobile communication links are susceptible to the effects of multi-path fading in channel. This causes distortion of the phase of the communication signal. This is particularly problematic in the case of continuous phase modulation (CPM) signals; wherein the information is contained in the phase of the signal. Pilot symbols are symbols that are known a-priori to the receiver that are periodically inserted by the transmitter in the transmitted sequence to aid the receiver to estimate the phase distortion caused by the channel. In the case of continuous phase modulation (CPM), every pilot location needs to have a symbol to bring the phase state to a xe2x80x9cknownxe2x80x9d state. This is generally referred to as the xe2x80x9cnullxe2x80x9d symbol. A typical null-pilot-symbol aided continuous phase modulation system is taught by Ho et al. in U.S. Pat. No. 5,712,877 and is herein incorporated by reference. Ho et al. teaches an apparatus for transmitting and receiving digital information using a pilot symbol insertion device for periodically inserting data dependent pilot symbols into frames of digital data for subsequent channel phase distortion estimation.
One solution for fast acquisition enables the operation of a transmit interrupt feature that is one of the distinguishing features of carrier phase modulation (CPM) used with the new Digital Interchange of Information and Signaling (DIIS) standard that is intended to enable the transition from the analog technology in today""s low tier Private Mobile Radio(PMR) systems. This type of system enables a higher speed (12 Kbps) digital communication supporting both speech and data. This is an evolution from an earlier European standard, Binary Interchange of Information and Signaling (BIIS) also known as ETS300.230.PMR protocol (DIIS).
The operation of a sync acquisition system depends on a known sequence of thirty symbols that is periodically (once every 720 ms) embedded in the transmitted symbol bit stream. This sequence of symbols, already known to the receiver, is called the synchronization word. Any subsequent call related information is generally sent immediately after the sync word. In this way, any receiver when establishing initial communication, starts looking for the sync word and call information to decide whether to participate in the communication or xe2x80x9ccallxe2x80x9d. Additionally, pilot symbols are inserted in the data stream to aid the receiver to estimate the channel phase distortion. The pilot symbols are inserted much more frequently (once every 20 ms) than the sync word. Therefore a pilot symbol based AFC gets many more estimates of the frequency error than a sync word based frequency control for fast and accurate frequency correction.
Coherent demodulation requires the knowledge of frequency and phase of the received signal. Even with the same nominal frequencies, there is always a difference between the actual frequency of oscillators of the transmitting and receiving radios. Automatic frequency correction (AFC) is used to estimate and correct this frequency offset in the received signal. It is necessary to correct the frequency offset in as short an amount of time as possible to a high degree of accuracy. Thus, it is necessary to address the problem of fast acquisition of frequency synchronization. This issue becomes much more significant in case of a late entering radio, where a call is already in progress. The time spent in acquiring frequency synchronization implies additional loss of symbols. This fast frequency acquisition becomes crucial.
The functional diagram of a typical digital receiver may be similar to the one shown in prior art FIG. 1. A common issue associated with this type of receiver is acquisition time. Acquisition time is the time it takes to sync transmitted data with received data i.e. the time during which the receiver cannot receive data since it is not yet in sync with the transmitted data. Digital in-phase (I) and quadrature (Q) baseband (zero center frequency or low IF or very low IF) signals 102 are input to a coarse automatic frequency control (AFC) 104 for bringing the range of the radio frequency (RF) input signal within the range of a sharp digital channel select (CS) filter 106.
Although the CS filter typically has a 3-dB bandwidth at 3 KHz for the DIIS modulation, such CS filter is chosen to select the desired signal while rejecting any off-channel power. Without the coarse AFC 104 however, the digital signal might be shifted out of the CS passband in view of the frequency. Typically for DIIS modulation it is required to bring the digital I-Q input signal 102 within 600 Hz of the center frequency of the CS filter 106 or too much signal is lost.
The filtered signal is then passed to frame sync detector 108 which is a device looking for a sequence of digital symbols that is known to the receiver apriori. Thus anytime the receiver detects energy within the IF filter passband, it begins the process of detecting a known sequence of bits for frame symbolization. By using the fine symbol time estimator 110, the receiver determines the boundary between symbols and also achieves frame synchronization (i.e. recognizes the known pattern of incoming bits of information).
Based on the time symbol estimation the receiver 100 will next do a fine frequency estimation to further reduce the frequency error between the transmitter and receiver frequencies. In order to properly decode data it is necessary to make this frequency error smaller than the tolerance of the symbol detection scheme. The tolerance could be as small as 10 Hz in case of coherent detection of DIIS signal or 100 Hz for non-coherent detection of DIIS signal. Since time synchronization has already been achieved, the fine frequency estimation works on known symbols using a fine frequency estimator 112. Since the coarse AFC 104 can only tune the incoming I-Q baseband signal to within 600 Hz, the fine frequency estimator 112 works to fine tune the frequency of incoming data to approximately with 10 Hz in order to property detect the incoming data symbol. This correction is applied to mixer 114 where it is mixed with the signal from the IF filter 106. The output of the mixer 114 is then applied to the symbol detector 116 where it is then properly detected.
The prior art receiver synchronization system as seen in FIG. 1 has several weaknesses. The CS filter with a 3-dB bandwidth at 3 KHz is typically required for meeting an adjacent channel interference protection requirement. With this 3 dB bandwidth, a maximum offset of 600 Hz is acceptable at the input of the IF filter. According to related standards specifications, a mobile transmitter frequency is allowed to be up to 1.5 KHz away from its nominal value for a channel separation of 12.5 KHz. If the baseband I-Q signal is directly fed to the CS filter, in the worst case, with a difference of 3 KHz between Tx and Rx, a significant part of the desired signal gets attenuated by the CS filter. This accounts for the coarse AFC 104 placed before the CS filter 106. The coarse AFC 104 is supposed to bring the filter offset down form 3 KHz to 600 Hz. The coarse AFC 104 however has to operate on unknown data symbols before the sync word, for the sync word to pass through the IF filter. This ultimately leads to a greater than acceptable delay and a period in which no synchronization occurs where the receiver is unable to receive information. Furthermore the sync word based prior art can generate a new estimate for frequency error much less often (once every 720 ms).
Therefore the needs exists to provide a digital receiver synchronization system for continuous phase modulation that can easily and accurately provide frequency synchronization to an incoming data stream with minimal delay to prevent any loss of incoming digital information.